Braking system for trailers of utility vehicles

ABSTRACT

The brakes of the front axle of the trailer of a utility vehicle are impinged upon with a brake pressure by way of a single, common ABS valve. Said ABS valve is electrically actuated by an EBS module associated with the rear wheel brakes depending on a differential slip between the front axle and the rear axle.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a braking system for trailers ofutility vehicles which have a steerable front axle, including front axlebrake cylinders, rear axle brake cylinders, rotational wheel speedsensors and an ABS valve.

From German Patent Document DE 38 29 951 C2, a utility vehicle brakingsystem for the load-dependent brake pressure control is known in which asolenoid valve, corresponding to a conventional ABS valve, as well as arotational wheel speed sensor are assigned to each of the brakecylinders of the four vehicle wheels. The solenoid valves and therotational wheel speed sensors are connected with a central controlunit, which controls the solenoid valves during braking as a function ofthe axle load distribution. In the case of this system, there is nodetermination at all of the absolute wheel slips for theaxle-load-dependent brake pressure control. On the contrary, for theanalysis of the rotational wheel speed signals, the exceeding of arather rough interaxle “rotational wheel speed difference threshold” is,in each case, used as a criterion for the brake pressure limitation.

From German Patent Document DE 44 43 522 A1, a method is known fordetermining the incline of a road by which, for a powered drivingcondition and a free-rolling driving condition, in each case, arotational wheel speed difference of a powered axle and of a non-poweredaxle is determined and, from these differences, an incline constant isdetermined.

From German Patent Document DE 198 09 546 C1, a method for automaticallydetermining the wheel base of steerable vehicles during cornering isknown, in which case the wheel base is determined from a defined wheeltrack and measured wheel circumference speeds.

From the applicant's internal state of the art, an electronic brakingsystem for utility vehicle trailers is known, in which an activepressure control module is, in each case, assigned to the brakes of thefront axle and the brakes of the rear axle. These pressure controlmodules each have three solenoid valves, specifically for ventilatingand bleeding, and for a “back-up circuit”, as well as a pressure sensor.The controlling of the pressure control module of the front axle takesplace by way of the pressure control module of the rear axle, whichleads to high wiring expenditures. Specifically, three control lines andone grounding conductor for the solenoid valves, as well as a sensorline and a grounding conductor for the pressure sensor, are required forthis purpose.

It is an object of the present invention to provide a braking system forutility vehicle trailers which is cost-effective and requires lowerconstructional expenditures.

This object is achieved by providing a braking system for utilityvehicle trailers which have a steerable front axle, including front axlebrake cylinders, rear axle brake cylinders, rotational wheel speedsensors and an ABS valve. The ABS valve is assigned jointly to bothbrake cylinders and is provided for controlling brake pressure into thefront axle brake cylinders. An EBS module is assigned to the rear axlebrake cylinders for controlling brake pressure into the rear axle brakecylinders. An electric control output of the EBS module is connectedwith an electric control input of the ABS valve. The rotational wheelspeed sensors are connected to the EBS module for determining adifferential slip between the front axle and the rear axle. The EBSmodule controls the ABS valve as a function of the determineddifferential slip. Advantageous embodiments and further developments ofthe invention are contained in the subclaims.

It is the main principle of the invention to control the brakes of thefront axle of the utility vehicle trailer by a single joint ABS valve,which is assigned to the two front axle brakes and which is controlledby an electronic braking system, that is, by an EBS module, primarilyassigned to the rear axle brakes, specifically as a function of aso-called “differential slip” between the front axle and the rear axle.

More simply stated, the brake pressure control of the rear axle brakestakes place by means of a pressure control module which electricallycontrols the ABS valve assigned to the two front axle brakes.

The ABS relay valve consists only of two solenoid valves for the holdingor bleeding of brake pressure. A pressure sensor for the front axlebrakes is not required here. A back-up valve is also not provided. Thus,for connecting the pressure control module with the ABS valve, onlythree connection lines are required.

Since, according to the invention, no pressure sensor is provided at thefront axle, it is true that the pressure control at the front axlebrakes cannot be implemented quite as precisely as by means of apressure sensor. However, the “absence” of a pressure sensor at thefront axle can largely be compensated by a so-called “differential slipcontrol algorithm” because the wheel speeds are measured and wheel slipsat the wheels of both axles are determined therefrom, which permits theobtaining of “information” concerning the momentary braking powerdistribution.

The brake pressure control at the front axle therefore takes place as afunction of the rotational wheel speed signals supplied by rotationalwheel speed sensors and a “differential slip signal” between the frontaxle and the rear axle derived therefrom.

The object of the control consists of controlling the front axle brakepressure such that the slip occurring during a braking at the frontwheels and the rear wheels is identical, or that the difference betweena “slip signal” of the front axle and of the rear axle is kept in adefined value range.

In this manner, an approximately uniform deceleration of both axles or auniform deceleration ratio is ensured. For this purpose, the front axlebrake pressure is modulated correspondingly; that is, the brake pressuredefined by way of the pneumatic control line is reduced, as required.The reason is that the brake pressure defined by way of the pneumaticcontrol line corresponds to the brake pressure which can be maximallycontrolled by way of the ABS valve into the front axle brake cylinders.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be explained in detail by means ofan embodiment in connection with the drawing.

FIG. 1 is a schematic representation of a braking system according tothe invention; and

FIG. 2 is a schematic representation of kinematic relationships of thetrailer vehicle during cornering.

DETAILED DESCRIPTION OF THE DRAWINGS

The utility vehicle trailer has a steerable front axle with front wheels1, 2 and a rear axle with rear wheels 3, 4. Rotational wheel speedsensors 5–8 are in each case assigned to the front wheels 1, 2 and therear wheels 3, 4, and are connected by way of electric lines 9–12 withan electropneumatic brake pressure control module 13 (EBS module) whichis primarily assigned to the rear axle brakes. One brake 14–17 is ineach case assigned to the front wheels 1, 2 and the rear wheels 3, 4,which brake 14–17 can be applied by means of brake cylinders 18, 19 ofthe front axle or spring-loaded brake cylinders 20, 21 of the rear axle.

The braking system of the trailer vehicle can be connected by way ofthree connections, specifically a pneumatic supply line connection 22, apneumatic control line connection 23 and an electric control connection24, with the braking system of a traction vehicle.

The supply line connection 22 is connected by way of a return valve 25and a parking valve 26 with an air brake reservoir 27. From the airbrake reservoir 27, a pneumatic line 28 leads to a supply input of thepressure control module 13. In addition, a pneumatic line 29 branchesoff the parking valve 26 to the pressure control module 13. A pneumaticline 30 extending between the parking valve 26 and the air brakereservoir 27 is connected with a supply input 31 of an ABS valve 32.

The ABS valve 32 is assigned jointly to both brake cylinders 18, 19 ofthe front axle and is connected with the brake cylinder 18 by way of apneumatic line 33 and with the brake cylinder 19 by way of a pneumaticline 34. The ABS valve 32 has two electric control inputs which areconnected by way of “one” electric line 35 shown here only schematicallywith the pressure control module 13.

Furthermore, the ABS valve 32 has a pneumatic control input 36 which isconnected by way of a return valve 37 with the pneumatic controlconnection 23. The pneumatic control input 36 is also connected by wayof a pneumatic control line 38 with a pneumatic control input of thepressure control module 13. The pressure control module 13 has anintegrated pressure sensor (not shown) which measures the pressure inthe pneumatic control line 38, that is, the control pressure present atthe pneumatic control input 36 of the ABS valve, which control pressureis identical to the maximal pressure which can be controlled into thebrake cylinders 18, 19.

The pressure control module 13 has pneumatic outputs 39–42 which areconnected by way of assigned pneumatic lines with the spring brakecylinders 20 or 21.

Furthermore, pneumatic axle load sensors or air bellows 43 44 areprovided at the rear axle and permit a determination of the axle load,particularly of the dynamic axle load during braking and starting. Theaxle load sensors 43, 44 are connected by way of electric lines with thepressure control module 13 which is shown here only as an example bymeans of the electric line 55. Correspondingly axle load sensors 45, 46may be provided at the front axle which, however, is not absolutelynecessary.

During a braking operation, the driver defines a braking demand signalby way of the brake pedal and the pneumatic control line 23 as well asthe electric control line 24. Corresponding to the braking demand by thedriver, the pressure control module 13 controls brake pressures into thespring brake cylinders 20, 21 as a function of the momentary axle loaddistribution. By way of the rotational wheel speed sensors 7, 8, therotational wheel speed behavior of the rear wheels 3, 4 is monitoredand, in the event of a locking risk, the rear wheel brake pressure islimited if required.

Simultaneously, the pressure control module 13 controls the ABS valve 32by way of the electric line 35, specifically as a function of the rearaxle brake pressure which is controlled into the spring brake cylinders20, 21, and the “rotational wheel speed behavior” of the entire vehicle,which is constantly monitored by way of the rotational wheel speedsensors 5–8. From the rotational wheel speed signals supplied by therotational wheel speed sensors 5–8, the pressure control module 13determines a “differential slip signal”, which supplies informationconcerning a possibly existing “slip difference” at the wheels 1, 2 and3, 4, respectively, of the front and rear axle.

During a braking operation, the pressure control module 13 controls theABS valve 32 such that an approximately identical wheel slip occurs atthe front axle and at the rear axle, or that the slip difference betweenthe front and rear axle is within a defined value range.

More simply stated, a front axle brake pressure is controlled into thebrake cylinders 18, 19 by the ABS valve 32, specifically as a functionof the braking demand defined by the driver as well as of the rear axleload. In a supplementary manner, by way of the axle load sensors 45, 46,the front axle load can be measured and included in the controlling ofthe ABS valve 32 or in the brake pressure control of the front axle.

The differential slip control according to the invention, essentiallyconsists of the following three main components:

-   1. Estimation or calculation of the actual brake pressure at the    front axle.-   2. Determination of the differential slip between the front axle and    the rear axle while taking into account a possible cornering.-   3. Controlling or regulating of the differential slip.    Estimation of the Front Axle Brake Pressure

It was mentioned above that no pressure sensor is provided at the frontaxle. The brake pressure at the front axle is therefore not measuredprecisely but estimated, that is, calculated in an approximate manner.

The control pressure is applied to the ABS valve 32 by way of thepneumatic control input 36. This control pressure is known because it ismeasured by a pressure sensor (not shown) in the pressure control module13. The pressure range, in which the estimated output pressure of theABS valve 32, that is, the brake pressure of the front axle, may besituated, is between zero and the control pressure existing in the lines36, 38, which is measured in the pressure control module 13.

The actually controlled-in front axle pressure is calculated in anapproximate manner by a time-related integration, specifically from the“controlling characteristic” of the ABS valve 32 defined by the pressurecontrol module 13 by way of the electric line 35, that is, the numberand time duration of the “opening and closing cycles” of the ABS valve,and the momentarily existing control and supply pressure, the supplypressure being, for example, 8 bar.

The pressure control module therefore supplies the brake cylinders 18,19 by way of the ABS valve 32 with a “desired” front axle brakepressure. For estimating the pressure, a “variable” is used whichspecifies the estimation range, in which case “normally” the controlpressure defined by way of the pneumatic control line 23, which is equalto the momentary maximally controllable input front-axle brake pressure“P MAX”, is used as the range argument. However, when the front axlevalve of a 4S3M arrangement (braking system with four sensors and threebrake pressure control valves) is considered and the EBS is active andthe demanded brake pressure is higher than a threshold value of thepressure control module, the following applies:

-   -   The demanded brake pressure is to be used as a range limit for        the estimation;    -   the desired front axle brake pressure is to be used as a “range        limit” for the ABS;    -   the minimum from the desired front axle brake pressure and the        desired ABS brake pressure is to be used as the desired pressure        for the estimation.        Determination of the Differential Slip

By means of the differential slip calculation, the following data areobtained for processing by a “differential slip controller”:

-   1. A signal indicating that the wheel base—track ratio of the    vehicle has been “learned”;-   2. the momentary “cornering ratio”;-   3. the diameter-adjusted and cornering-ratio-adjusted differential    slip.

The wheel base—track ratio of the utility vehicle trailer with asteerable front axle can be “learned” or calculated from the wheelspeeds if certain conditions are met. The signal indicating that thewheel base—track ratio was learned will be generated when the wheelbase—track ratio has been learned at least once since being switched-on(power on).

The wheel base—track ratio can be stored in an EEPROM. In this case, theabove-mentioned signal will be generated when the wheel base—track ratiohas been learned at least once since the installation of the EBS systemof the trailer vehicle. The signal is to be canceled only when a wheelbase—track ratio has been learned which differs significantly from thevalue stored in the EEPROM. This may indicate that the EBS module of thetrailer vehicle was installed in another vehicle.

When the signal is generated, the “slip controller” is to rely on thecornering-compensated differential slip value even during narrowcornering.

The signal is to be set during an ABS intervention and additionally 0.5s afterwards.

Cornering Ratio

The cornering ratio supplies information on how narrowly the vehicle iscornering at the time. Here, the cornering ratio is the amount-relatedratio of the difference between the two rear wheel speeds of the vehicleand the “mean” rear wheel speed. It is indicated in percent and is anon-negative value (amount). The reason is that the cornering ratio doesnot have to supply information on whether the vehicle is just corneringin the left or the right direction.

Differential Slip

The differential slip is “wheel-diameter-compensated andcornering-compensated”. This means that, during the monitoring ofwhether a “slip difference” exists between the front axle and the rearaxle, the wheel diameters of the vehicle and the cornering ratio have tobe taken into account, particularly whether the vehicle is momentarilycornering or is driving straight ahead. The reason is that, in the caseof a trailer vehicle with a steerable front axle, during a cornering,the front wheel speeds are per se greater than the speeds of the rearwheels. The slip difference can therefore not be determined by a“simple” wheel speed difference formation between the front axle and therear axle. If, during the determination of the differential slip, themomentary cornering ratio were not taken into account, thus, nodifferentiation were made between a cornering and a straight-aheaddrive, errors would necessarily occur.

The kinematic relationships in the case of a trailer vehicle with asteerable front axle will be explained in detail in connection with FIG.2. In this case, the following variables will be used:

Speeds of the front wheels v1, v2 speeds of the rear wheels v3, v4 meanfront axle speed v_(VA) mean rear axle speed v_(HA) speed difference ofrear wheels Δv_(HA) reference speed of front axle v_(VA,Ref) wheel baseRST track SP wheel base - track ratio Φ yaw rate of trailer vehicledΩ/dt mean cornering radius of front axle R1 relative to theinstantaneous center of rotation M mean cornering radius of rear axle R2relative to the instantaneous center of rotation M

FIG. 2 is a schematic view of a trailer vehicle with a steerable frontaxle with front wheels 1, 2 and rear wheels 3, 4 during a cornering. Bymeans of the rotational wheel speed sensors 5, 6 illustrated in FIG. 1,the wheel circumference speeds v1, v2 of the front wheels 1, 2 aremeasured. Correspondingly, the rotational wheel speed sensors 7, 8measure the rotational wheel speeds, or the wheel circumference speedsv3, v4 of the rear wheels 3, 4 are measured while taking into accountthe wheel diameters. From the wheel circumference speeds v1, v2, a“mean” front axle speed v_(VA) can be determined:

$v_{VA} = \frac{{v1} + {v2}}{2}$

Correspondingly, a mean rear axle speed v_(HA) is determined:

$v_{HA} = \frac{{v3} + {v4}}{2}$

The differential slip Δs between the front axle and the rear axle can bedefined as follows:

${\Delta\; s} = {\frac{v_{VA} - v_{HA}}{v_{HA}}\mspace{14mu}\lbrack {0.1\mspace{14mu}\%} \rbrack}$Determination of the Wheel Base—Track Ratio

By using the Pythagorean theorem, the wheel base can be calculated fromthe mean front axle speed and the mean rear axle speed.

The wheel base of the trailer vehicle is therefore:RST=√{square root over (R1 ² −R2 ² )}

When the two mean cornering radii are each multiplied by the yaw rate ofthe vehicle, the “tangential speeds” are obtained at the axle centerpoints, that is, the mean front axle speed v_(VA) and the mean rear axlespeed v_(HA):v _(HA) =dΩ/dt≅R1;v _(HA) =dΩ/dt≅R2.

The differential speed at the rear axle, that is, the speed differencebetween the right rear axle wheel 3 and the left rear axle wheel 4 is:Δv _(HA) =dΩ/dt≅SP.

The wheel base—track ratio Φ can be calculated as follows:

$\phi = {\frac{RST}{SP} = \frac{\sqrt{{R1}^{2} - {R2}^{2}}}{SP}}$

When a multiplication takes place by means of dΩ/dt, the following isobtained:

$\phi = {\frac{\sqrt{( {\frac{\mathbb{d}\Omega}{\mathbb{d}t} \cdot {R1}} )^{2} - ( {\frac{\mathbb{d}\Omega}{\mathbb{d}t}\; \cdot {R2}} )^{2}}}{\frac{\mathbb{d}\Omega}{\mathbb{d}t}\; \cdot {SP}} = \frac{\sqrt{v_{VA}^{2} - v_{HA}^{2}}}{\Delta\; v_{HA}}}$

The wheel base—track ratio can therefore be calculated from the wheelspeeds.

However, this equation applies only if the following marginal conditionshave been met:

-   a) No longitudinal slip at the wheels or no braking;-   b) no lateral slip at the wheels, that is, no significant lateral    acceleration;-   c) the wheel speeds are corrected by a wheel diameter calibration;-   d) the wheel speed values are not too low for achieving a sufficient    accuracy;-   e) a significant cornering ratio exists for a sufficient accuracy.

The lateral acceleration can be determined from the speeds and thecornering ratio. The learning operation for the wheel base—track ratioshould be based on an average measuring duration of one second, duringwhich the above-mentioned conditions are continuously met.

When the learning operation has been implemented at least once afterbeing switched-on (power on), the signal should be set which indicatesthe learning operation. Until then, a defined value (default value)should be used for the wheel base—track ratio. For example, Φ=2.5 may bedefined as an “initial value” for the wheel base—track ratio.

Calculation of the Differential Slip Between the Front Axle and the RearAxle

The calculation may be based on the above-mentioned formula

${\Delta\; s} = \frac{v_{VA} - v_{HA}}{v_{HA}}$

In this formula, the mean rear axle speed v_(HA) is the reference speed(denominator).

Instead of the mean rear axle speed v_(HA), however, a “front axlereference speed” should be used in the denominator in practice, whichfront axle reference speed is calculated from the mean rear axle speedv_(HA), the cornering ratio and the wheel base—track ratio.

FIG. 2 indicates that, when the Pythagorean theorem is used, the frontaxle reference v_(VA,Ref) can be calculated as follows:v _(VA,Ref) =√{square root over (v _(HA) ² +(dΩ/dt·RST) ² )}v _(VA,Ref) =√{square root over (v _(HA) ² +(dΩ/dt·SP·φ) ² )}

With Δv_(HA)=dΩ/dt≅SP, the following is obtained:v _(VA,Ref) =√{square root over (v _(HA) ² +(Δv _(HA) ·φ) ² )}

Thus, the formula for calculating the differential slip Δs is asfollows:

${\Delta\; s} = {\frac{v_{VA} - v_{{VA},{Ref}}}{v_{{VA},{Ref}}} = {\frac{v_{VA}}{V_{{VA},{Ref}}} - {1\mspace{14mu}\lbrack {0.1\mspace{14mu}\%} \rbrack}}}$

The differential slip value should always be filtered, the time constantbeing, for example, 0.25 s. In the event of an ABS intervention,however, no filtering should take place because excessively highdifferential slip values during an ABS intervention would influence thevalues after the ABS intervention.

Slip Controller

The following quantities are fed to the slip controller as inputquantities:

-   -   Compensated differential slip ([0.1%];    -   cornering ratio [%];    -   signal indicating that the wheel base—track ratio has been        learned.

The differential slip controller supplies a desired front axle pressurep_(VA) as an output signal, or a control signal corresponding to thisdesired front axle pressure for the ABS valve of the front axle.

The slip controller is a pure integral controller (I controller). Thefeedback signal for the slip controller is the compensated differentialslip Δs [0.1%].

The control signal or the control variable of the controller is theratio between the front axle brake pressure and the trailer brakepressure; that is, the rear axle brake pressure demanded by the driverin [0.1%].

${{Control}\mspace{14mu}{signal}} = {\frac{p_{VA}}{p_{HA}}\mspace{14mu}\lbrack {0.1\mspace{14mu}\%} \rbrack}$

A control signal of 1,000 means that the front axle brake pressurep_(VA) and the rear axle brake pressure p_(HA) are the same.

The amplification of the I term, which is the control signal itself,should be

$\frac{30\mspace{14mu}{\%/s}}{\%}$

This means that, in the case of a differential slip of +1%, the controlsignal, that is, the relationship between the desired front axle brakepressure p_(VA) and the desired rear axle brake pressure p_(HA) has arise rate of 30%/s.

The controller should have a tolerance band of 1%. This means that, ifthe differential slip is in the interval [−1%, +1%], which correspondsto differential slip values of −10 . . . +10, the amplification shouldbe canceled, that is, the control signal should not be changed.

Initialization of the Controller

After an operation of the EBS system, the control signal (I term) shouldbe initialized, specifically at a defined ratio of 1:1, whichcorresponds to a “starting value” of 1,000.

However, this starting value can be adapted during several brakeapplications. The optimal braking power distribution depends on thegeometrical dimensions of the trailer and on the horizontal and verticalposition of the center of gravity, which is a function of the load. Noneof these characteristics changes rapidly from one moment to the next. Itis therefore assumed that the load condition can change exclusivelybetween a switched-off condition and a switched-on condition or when thevehicle is stopped in the switched-on condition, if this condition doesnot last longer than 5 minutes.

A storage variable for storing the initialization value of the controlsignal should be provided. The default value after a “reset” should be1,000. It should be overwritten by the control signal when the followingconditions are met:

-   a) The wheel speeds are modified by a wheel diameter calibration;-   b) the wheel diameter calibration has been terminated at least once    since the switching-on;-   c) the EBS and the slip control are active;-   d) no ABS intervention has taken place during the momentary brake    application;-   e) the differential slip was constantly between −1% . . . +1% during    the last 0.5 s;-   f) the cornering ratio is lower than 3%.

The initialization value should be reset to the default value of 1,000after a 5-minute vehicle stoppage.

Safety Level

The braking system monitors whether the required conditions exist for acomplete operation. The following are necessary conditions:

-   -   Four rotational wheel speed signals exist;    -   the control pressure signal is present;    -   the control of the rear axle brake pressure is available.

The following safety levels are required:

-   -   Complete functioning when everything is available;    -   uniform braking power distribution; that is, no slip control        when a disturbance is present during the wheel sensing;    -   control pressure is controlled through to the outputs when a        disturbance is present in the rear axle brake pressure control;        in this case, a pressure control takes place at none of the        axles.        a) Control Off

When a disturbance occurs in the EBS system, the ABS valve controls themaximal value defined by way of the control pressure line into the frontaxle brake cylinders.

b) 1:1 Control

In the case of a disturbance of a rotational wheel speed sensor or whena rotational wheel speed sensor does not operate reliably, a“switch-over” takes place to a “default” condition when the EBS systemis not active. In this case, the output signal, that is, the front axlebrake pressure p_(VA) is identical with the rear axle brake pressurep_(HA).

c) Control Active

The front axle brake pressure p_(VA) should be:

$p_{V\; A} = {p_{HA} \cong \frac{\text{control~~signal}}{1000}}$

The control signal should be calculated in each cycle. In the case of anEBS intervention, the “default condition” should be taken up (only inthe case of a 4S3M arrangement).

d) Control Frozen

This condition corresponds essentially to the active condition, but thecontrol signal is not changed. The braking power distribution istherefore “frozen”.

A transition takes place from the “active condition” to the “frozen”condition if one of the following conditions is met:

-   -   No ABS flag clear (ABS activity and the following 5 s failed);    -   cornering ratio>lower limit (when the wheel base—track ratio was        not learned);    -   cornering ratio>upper limit (when the wheel base—track ratio was        learned).

1. Braking system for a utility vehicle trailer having a steerable frontaxle, the system comprising: front axle brake cylinders, rear axle brakecylinders, and rotational wheel speed sensors; an ABS valve assignedjointly to both front axle brake cylinders for controlling brakepressure into the front axle brake cylinders; an EBS module assigned tothe rear axle brake cylinders for controlling brake pressure into therear axle brake cylinders; an electric control output of the EBS modulecoupled with an electric control input of the ABS valve; wherein therotational wheel speed sensors are connected to the EBS module fordetermining a differential slip (Δs) between the steerable front axleand a rear axle of the trailer; wherein the EBS module controls the ABSvalve as a function of the determined differential slip (Δs); andwherein the differential slip (Δs) is determined from a mean front axlespeed (v_(VA)) and a front axle reference speed (v_(VA,Ref)), accordingto the formula:${\Delta\; s} = {\frac{v_{VA} - v_{{VA},{Ref}}}{v_{{VA},{Ref}}} = {\frac{v_{VA}}{v_{{VA},{Ref}}} - 1.}}$2. Braking system according to claim 1, wherein for controlling the ABSvalve, the EBS module determines a desired front axle brake pressure(p_(VA)) as a function of a control pressure defined by a driver, thecontrol pressure being defined by the driver by way of a pneumaticcontrol line, or a momentarily controlled-in rear axle brake pressure(p_(VA)) and a momentary differential slip (Δ_(VA)).
 3. Braking systemaccording to claim 2, wherein the ABS valve or the desired front axlebrake pressure (p_(VA)) are controlled such that the same slip occurs atthe steerable front axle and at the rear axle or that the differentialslip (Δs) is held in a defined permissible value range.
 4. Brakingsystem according to claim 1, wherein the front axle reference speed(V_(VA,Ref)) is determined from a mean rear axle speed (v_(HA)), a speeddifference between rear wheels on the rear axle (Δv_(HA)) and a wheelbase—track ratio (Φ) of the trailer vehicle according to the formula:v _(VA,Ref) =√{square root over (v _(HA) ² +Δv _(HA) ·φ) ² )}, 5.Braking system according to claim 4, wherein the wheel base—track ratio(Φ) of the trailer is determined from the mean front axle speed(v_(VA)), the mean rear axle speed (v_(HA)), and the speed difference ofthe rear wheels (Δv_(HA)) according to the formula:$\phi = {\frac{\sqrt{v_{VA}^{2} - v_{HA}^{2}}}{\Delta\; v_{HA}}.}$ 6.Braking system according to claim 5, wherein an initial value of Ø=2.5is defined for the wheel base—track ratio (Φ) of the trailer.
 7. Brakingsystem according to claim 2, wherein the ABS valve has a control inputcoupled with a pneumatic control line, and further wherein, in the caseof a disturbance of the EBS module, the ABS valve controls a brakepressure defined by the driver by way of the pneumatic control line intothe front axle brake cylinders.
 8. Braking system according to claim 2,wherein in the case of a disturbance of one or more of the rotationalwheel speed sensors, the ABS valve is controlled such that the frontaxle brake pressure (p_(VA)) is identical with the rear axle brakepressure (p_(VA)) controlled in by the EBS module.
 9. Braking system fora utility vehicle trailer having a steerable front axle, the systemcomprising: front axle brake cylinders, rear axle brake cylinders, androtational wheel speed sensors; an ABS valve assigned jointly to bothfront axle brake cylinders for controlling brake pressure into the frontaxle brake cylinders; an EBS module assigned to the rear axle brakecylinders for controlling brake pressure into the rear axle brakecylinders; an electric control output of the EBS module coupled with anelectric control input of the ABS valve; wherein the rotational wheelspeed sensors are connected to the EBS module for determining adifferential slip (Δs) between the steerable front axle and a rear axleof the trailer; wherein the EBS module controls the ABS valve as afunction of the determined differential slip (Δs); and wherein the ABSvalve has a control input coupled with a pneumatic control line, andfurther wherein, in the case of a disturbance of the EBS module, the ABSvalve controls a brake pressure defined by a driver by way of thepneumatic control line into the front axle brake cylinders.
 10. Brakingsystem according to claim 9, wherein the ABS valve or a desired frontaxle brake pressure (p_(VA)) are controlled such that the same slipoccurs at the steerable front axle and at the rear axle or that thedifferential slip (Δs) is held in a defined permissible value range. 11.Braking system according to claim 9, wherein the differential slip (Δs)is determined from a mean front axle speed (v_(VA)) and a front axlereference speed (v_(VA,Ref)), according to the formula:${\Delta\; s} = {\frac{v_{VA} - v_{{VA},{Ref}}}{v_{{VA},{Ref}}} = {\frac{v_{VA}}{v_{{VA},{Ref}}} - 1.}}$12. Braking system according to claim 11, wherein the front axlereference speed (v_(VA,Ref)) is determined from a mean rear axle speed(v_(HA)), a speed difference between rear wheels on the rear axle(Δv_(HA)) and a wheel base—track ratio (Φ) of the trailer vehicleaccording to the formula:v _(VA,Ref) =√{square root over (v _(HA) ² +Δv _(HA) ·φ) ² )}, 13.Braking system according to claim 12, wherein the wheel base—track ratio(Φ) of the trailer is determined from the mean front axle speed(v_(VA)), the mean rear axle speed (v_(HA)), and the speed difference ofthe rear wheels (Δv_(HA)) according to the formula:$\phi = {\frac{\sqrt{v_{VA}^{2} - v_{HA}^{2}}}{\Delta\; v_{HA}}.}$ 14.Braking system according to claim 13, wherein an initial value of Ø=2.5is defined for the wheel base—track ratio (Φ) of the trailer. 15.Braking system for a utility vehicle trailer having a steerable frontaxle, the system comprising: front axle brake cylinders, rear axle brakecylinders, and rotational wheel speed sensors; an ABS valve assignedjointly to both front axle brake cylinders for controlling brakepressure into the front axle brake cylinders; an EBS module assigned tothe rear axle brake cylinders for controlling brake pressure into therear axle brake cylinders; an electric control output of the EBS modulecoupled with an electric control input of the ABS valve; wherein therotational wheel speed sensors are connected to the EBS module fordetermining a differential slip (Δs) between the steerable front axleand a rear axle of the trailer; wherein the EBS module controls the ABSvalve as a function of the determined differential slip (Δs); andwherein in the case of a disturbance of one or more of the rotationalwheel speed sensors, the ABS valve is controlled such that a front axlebrake pressure (p_(VA)) is identical with the rear axle brake pressure(p_(VA)) controlled in by the EBS module.
 16. Braking system accordingto claim 15, wherein the differential slip (Δs) is determined from amean front axle speed (v_(VA)) and a front axle reference speed(v_(VA,Ref)), according to the formula:${\Delta\; s} = {\frac{v_{VA} - v_{{VA},{Ref}}}{v_{{VA},{Ref}}} = {\frac{v_{VA}}{v_{{VA},{Ref}}} - 1.}}$17. Braking system according to claim 16, wherein the front axlereference speed (v_(VA,Ref)) is determined from a mean rear axle speed(v_(HA)), a speed difference between rear wheels on the rear axle rearwheels (Δv_(HA)) and a wheel base—track ratio (Φ) of the trailer vehicleaccording to the formula:v _(VA,Ref) =√{square root over (v _(HA) ² +Δv _(HA) ·φ) ² )}, 18.Braking system according to claim 17, wherein the wheel base—track ratio(Φ) of the trailer is determined from the mean front axle speed(v_(VA)), the mean rear axle speed (v_(HA)), and the speed difference ofthe rear wheels (Δv_(HA)) according to the formula:$\phi = {\frac{\sqrt{v_{VA}^{2} - v_{HA}^{2}}}{\Delta\; v_{HA}}.}$ 19.Braking system according to claim 18, wherein an initial value of Ø=2.5is defined for the wheel base—track ratio (Φ) of the trailer.
 20. Abraking method for a utility vehicle trailer having a steerable frontaxle, the braking method comprising the acts of: controlling brakepressures into front axle brake cylinders via a single joint ABS valve;controlling brake pressures into rear axle brake cylinders via an EBSmodule, said EBS module being electrically coupled with the ABS valve;determining a differential slip value between the steerable front axleand a rear axle via the EBS module; controlling the ABS valve via theEBS module as a function of the determined differential slip value; andwherein for controlling the ABS valve, the EBS module determines adesired front axle brake pressure (p_(VA)) as a function of a controlpressure defined by a driver, the control pressure being defined by thedriver by way of a pneumatic control line, or a momentarilycontrolled-in rear axle brake pressure (p_(VA)) and a momentarydifferential slip (Δ_(VA)).
 21. Braking method according to claim 20,wherein the ABS valve or the desired front axle brake pressure (p_(VA))are controlled such that the same slip occurs at the front axle and atthe rear axle or that the differential slip (Δs) is held in a definedpermissible value range.
 22. A braking method for a utility vehicletrailer having a steerable front axle, the braking method comprising theacts of: controlling brake pressures into front axle brake cylinders viaa single joint ABS valve; controlling brake pressures into rear axlebrake cylinders via an EBS module, said EBS module being electricallycoupled with the ABS valve; determining a differential slip valuebetween the steerable front axle and a rear axle via the EBS module;controlling the ABS valve via the EBS module as a function of thedetermined differential slip value; and wherein the differential slip(Δs) is determined from a mean front axle speed (v_(VA)) and a frontaxle reference speed (v_(VA,Ref)), according to the formula:${\Delta\; s} = {\frac{v_{VA} - v_{{VA},{Ref}}}{v_{{VA},{Ref}}} = {\frac{v_{VA}}{v_{{VA},{Ref}}} - 1.}}$23. Braking method according to claim 22, wherein the front axlereference speed (v_(VA,Ref)) is determined from a mean rear axle speed(v_(HA)), a speed difference between rear wheels on the rear axle(Δv_(HA)) and a wheel base—track ratio (Φ) of the trailer vehicleaccording to the formula:v _(VA,Ref) =√{square root over (v _(HA) ² +Δv _(HA) ·φ) ² )}, 24.Braking method according to claim 23, wherein the wheel base—track ratio(Φ) of the trailer is determined from the mean front axle speed(v_(VA)), the mean rear axle speed (v_(HA)), and the speed difference ofthe rear wheels (Δv_(HA)) according to the formula:$\phi = {\frac{\sqrt{v_{VA}^{2} - v_{HA}^{2}}}{\Delta\; v_{HA}}.}$ 25.Braking method according to claim 24, wherein an initial value of Ø=2.5is defined for the wheel base—track ratio (Φ) of the trailer.